There are many things that can happen when keeping fijian iguanas in captivity. I will try and list the once i find relevant.

MBD( metabolic bone disease ).

The most common cause of mbd is from a imbalance in the levels of calcium, phosphorous, and vitamin D in their bodies.

The symptoms are often: Limping. Bowed legs. Hard lumps along the legs, spinal column, or jaw.

Softening and unusual flexibility of the lower jaw. Difficulty raising the body off the ground

Decreased appetite.

If caught and identified in the early stages, mbd is easy to treat. It can be treated with the use of a calcium/mineral mix( with vitamin D) that is mixed in water and given oral to the iguana. For a adult fiji iguana you will treat it for 3-4 days 2-3 times a day. For each treatment you will give the iguana about 0,5-1ml. The mix should look like whip cream before whipping it. This treatmet together with a quallity uv source, normally cure the mbd within those 3-4 days.

egg binding.

This is common in female fijian iguanas( its not a problem i personally have ever seen with my own iguanas).

Reasons for egg binding can be :

An anatomical defect in the female causing an obstructive dystocia, which makes it physically impossible for the egg to pass

An overly large or malformed egg, or one that is not positioned correctly, is broken, or joined to other eggs (fractured)

Poor condition of the mother (e.g., inactivity can lead to poor muscle tone and the inability to produce strong, repeated contractions). A female may become exhausted before she completes laying of all the eggs

Malnutrition, especially if resulting in hypocalcemia (low blood levels of calcium). Obesity may also be a cause of egg binding.

Dehydration.

Stress, such as overcrowding or too small of an enclosure.

Hormonal or disease conditions such as infections of the reproductive organs, dehydration, or kidney disease (kidneys may become enlarged and partially block the pelvic area.)

Breeding animals that are too young or too old, ill, or not in good condition. Excessive breeding of the same animal may also result in dystocia.

Dehydration.

Dehydration is a common problem. If they lose to much water there organs can shut down and death can occur.

This is why it is important to make sure the iguanas stay hydrated all the time. Correct hymidity and a mist system(not a fogger) is a good way to make sure the iguanas stay hydrated. Also the food offered can help the iguanas to stay hydrated, items like iceberg lettuce, cucumber and water melon are good sources of water.

These food items should not be used as a big part of the feeding.

Wounds.

Wounds sometime occurs when fijian iguanas are kept as pairs( specially in mating attempts). Females can often get wounds on the front legs or in there neck from the mating attempts, these can be flesh wounds. it is important to keep an eye on the wounds to make sure they dont get infected.

If there are signs of infection, You can use a normal desinfection cream( same as you use for other reptiles. there are different brands depending on where you live).

In cases of more serious infections contact a vet.

Shedding problms.

The most common reason for shedding problems is too low humidity. Humidity should stay around 70-80%. Sometimes the humidity is not enough, so it can be helpt with a spray bottle. Spray the iguana carefully to soak the skin it has problem shedding. I'm not a fan of giving them a bath, since it often stress them, so the spray bottle is in my opinion a better solution.